Immunizations

Encyclopedia of Public Health
COPYRIGHT 2002 The Gale Group Inc.

IMMUNIZATIONS

Immunization is the induction of immunity against an infectious disease by a means other than experiencing the natural infection. The term is usually used interchangeably with vaccination. Active immunization involves administration of an antigenic substance that then induces development of protective antibodies by the person immunized. This protection usually lasts for years, even for life. Passive immunization refers to temporary immunity resulting from antibodies developed by someone else, either through administration of immune globulin (e.g., gamma globulin, rabies immune globulin) or through the natural transfer across the placenta of antibodies developed by the mother, which provide protection to the newborn infant. Passive immunity usually lasts only a few weeks to a few months.

Substances used for active immunization include vaccines and toxoids. Vaccines may contain living, weakened (attenuated) organisms (measles), killed whole organisms (whole cell pertussis, influenza), portions of organisms (subunit influenza), purified components of organisms (acellular pertussis, pneumococcal polysaccharide), or they may be manufactured artificially (hepatitis B produced by recombinant DNA technology). For some diseases, vaccines may be available in more than one form (live attenuated and inactivated [killed] poliovirus vaccines, whole cell and acellular pertussis vaccines). Toxoids are made by preparing the toxins excreted by microorganisms and inactivating them physically or chemically. Diphtheria and tetanus are the most commonly used toxoids. Vaccines and toxoids may also contain adjuvants, substances that enhance the immune response, as well as preservatives.

Some vaccines (particularly live, attenuated vaccines) provide long-term, even lifelong protection following administration of only a single dose. Others (particularly inactivated vaccines and toxoids) may require administration of more than one dose in order to induce long-lasting immunity. Some vaccines (diphtheria, tetanus) require periodic booster doses in order to maintain immunity. Many vaccines may be inactivated by changes in temperature, particularly heat, and must be kept refrigerated or frozen from the time of manufacture until just before being administered. The need for this "cold chain" makes it difficult to carry out immunization programs in developing countries where refrigerators and freezers are not commonplace.

The rate of development of new vaccines has been accelerating as a result of improved knowledge of immunity and improvements in biotechnology. It was nearly one hundred years between Edward Jenner's first use of smallpox vaccine in 1796 and Louis Pasteur's development of the second vaccine (against rabies) in 1885. In the last twenty years of the twentieth century, many new or improved vaccines were developed and introduced, including vaccines directed against Haemophilus influenzae type b (Hib), hepatitis A, hepatitis B, Japanese encephalitis, meningococcal meningitis, pertussis, typhoid, and varicella (chicken pox). Dozens of other vaccines are under development.

Repeated economic analyses have shown that vaccines are among the most cost-effective health interventions available. For most of the vaccines used in infants and young children, the economic benefits of vaccination (avoidance of costs of medical care, hospitals, etc.) far outweigh the costs of vaccination, and the vaccines are truly cost saving. For others, the cost to prevent an illness or death is quite small and is substantially smaller than the cost to treat or cure the condition.

VACCINE RECOMMENDATIONS

In the United States, recommendations for vaccine use are made by the Public Health Service Advisory Committee on Immunization Practices, in conjunction with the American Academy of Pediatrics, American Academy of Family Practice, American College of Physicians (representing adult medicine specialists), and other professional organizations. Some vaccines are recommended for use in all persons (typically infants and young children, since most communicable diseases primarily strike them) and others are recommended for specific persons or groups who are at increased risk of contracting the particular disease. Vaccines currently recommended for use in all infants and children in the United States are DTP/DTaP (diphtheria and tetanus toxoids and pertussis [or acellular pertussis] vaccine), IPV (inactivated poliovirus vaccine), MMR (measles, mumps, and rubella vaccine), Hib vaccine (Haemophilus influenzae type b vaccine), hepatitis B vaccine, and varicella (chicken pox) vaccine. Several of these vaccines require more than one dose. The recommended schedule of immunizations in the year 2000 for infants and young children is shown in Figure 1.

Adolescents and adults also need vaccines, including MMR and hepatitis B if they have not already received them, as well as periodic boosters of tetanus and diphtheria toxoids. In addition, in the United States it is recommended that all persons sixty-five years of age or older receive a single dose of pneumococcal polysaccharide vaccine and annual doses of influenza vaccine because of the increased risk of complication or death if infected. Individuals younger than sixty-five who have chronic illnesses should also receive pneumococcal and influenza vaccines. Some vaccines recommended for persons at increased risk include yellow fever, hepatitis A, typhoid, meningococcal, and Japanese encephalitis vaccines for travelers to certain developing countries; rabies vaccine for veterinarians and persons working with potentially rabid animals; and hepatitis B vaccine for health care workers and others who might come in contact with body fluids.

VACCINE SAFETY

Although modern vaccines are safe and effective, they are neither perfectly effective nor perfectly safe. Some persons who have been vaccinated may still be susceptible to the disease, and some persons who receive the vaccine may suffer an adverse event caused by the vaccine. In developing a vaccine, major efforts are made to maximize effectiveness and minimize the risk of adverse events.

In determining whether to use a vaccine, it is necessary to balance the benefits of the vaccine against the risk of the disease and the risks from the vaccine. This balance may change over time.

Figure 1

For example, oral polio vaccine (OPV, Sabin vaccine) is made from live, attenuated polioviruses. Rarely, the person who receives the vaccine or someone who is in close contact with him or her may develop paralysis. Vaccine-associated paralysis occurs with a frequency of approximately one case for every million doses of OPV administered. By contrast, the inactivated polio vaccine (IPV, Salk vaccine) has no such risk of paralysis. However, OPV has advantages over IPV because it may be spread from the person who receives the vaccine to family members or other persons in contact with the vaccinee, thereby protecting them. Because it provides greater intestinal immunity than IPV, it protects against the spread of wild poliovirus if the vaccinated individual is exposed to wild poliovirus. The relative advantages of OPV have resulted in its being the vaccine chosen by virtually all countries of the world to control and eradicate polio. However, as the risk of wild poliovirus becomes smaller, the rare complications associated with OPV assume greater prominence. In the United States, the marked decline in risk of exposure to wild poliovirus as a result of global polio eradication efforts led in 1999 to a change in policy to favor use of IPV rather than OPV.

Assessment of adverse events associated with vaccines can be quite difficult. Pre-licensure trials typically involve a few thousand individuals and cannot be expected to detect reactions that occur with a frequency as low as (or lower than) one in 100,000. Consequently, it is important to maintain surveillance for adverse events after vaccines are licensed and introduced for widespread use. It may be very difficult to determine whether an event that occurs after vaccination was caused by the vaccine rather than occurring by chance, particularly if the event is known to occur in that age group. For example, sudden infant death syndrome (SIDS) is the leading cause of death in children two to four months of age. Since children typically receive DTP vaccine at two and four months, it is inevitable that on occasion a child will die of SIDS in the twenty-four hours following vaccination (or in the twenty-four hours preceding planned vaccination). The question is whether there is an increased incidence of SIDS following vaccination. Several studies have demonstrated that the incidence of SIDS is not increased following DTP vaccination.

Immunization provides protection both to the individuals immunized and to the community because immunized individuals do not transmit disease. If a high proportion of the population is immunized, the risk of exposure is reduced both for those who have not been immunized and those who have received vaccine but have not been protected. This "herd immunity" has led to the disappearance of disease in defined geographic areas, even though not everyone has received vaccine.

Introduction and widespread use of vaccines has had a dramatic effect on the occurrence of many diseases in the United States. Table 1 demonstrates the maximum number of cases of specified diseases ever reported in the United States, the number of cases reported in 1998, and the proportion reduction in incidence. Declines of greater than 95 percent are the rule. Similar dramatic reductions have been seen from deaths due to these diseases. Smallpox is not shown on this table as smallpox has been eradicated from the world. Most industrialized countries have seen comparable declines in illnesses and deaths due to vaccine-preventable diseases. Most developing countries have not yet experienced the same level of decline because they have not achieved the same level of immunization coverage.

In the United States, immunization levels in young children are at record highs and reported incidence of vaccine-preventable diseases are at record lows. Nonetheless, several factors threaten this continued success, including the birth every day of eleven thousand infants who will all need to be immunized, the changing immunization schedule, the movement of children between health care providers (25% of U.S. 2-year-olds have received vaccines from two or more providers), continued overestimation of coverage by parents and providers, and the absence of disease as a continuing reminder of the need for immunization (even though the causative organisms are still in circulation).

Because of the continuing birth of susceptible infants, unless communicable diseases are eradicated it will be necessary to continue immunizing

Table 1

Maximum Reported Morbidity and 1998 Provisional Morbidity

Vaccine-Peventable Diseases of Childhood

United States

Disease

Maximum Reported Morbidity

Provisional (1998) Morbidity

Decrease

*estimated

source: Centers for Disease Control and Prevention

Diphtheria

206,939

1

99.99%

Pertussis

265,269

6,279

97.63%

Tetanus

1,733

34

98.04%

Poliomyelitis (paralytic)

21,269

0

100%

Measles

894,134

89

99.99%

Mumps

152,209

606

99.60%

Rubella

57,686

345

99.40%

Congenital rubella syndrome

20,000*

6

99.97%

Haemophilus influenzae type b

20,000*

54

99.73%

against them indefinitely. Several examples exist in industrialized countries (including England and Japan) where epidemic resurgence of pertussis (whooping cough) has occurred as a consequence of declining use of pertussis vaccine. In the United States, a resurgence of measles resulted from the diversion of effort from measles vaccination to rubella vaccination following introduction of rubella vaccine in 1969 (at that time it was not combined with measles vaccine).

Several techniques have been demonstrated to be highly effective in improving and maintaining immunization coverage, including improving access to immunization, developing reminder and recall systems to notify parents and providers about needed or overdue immunizations, assessing immunization coverage in individual facilities, and linking immunization services with other services. By providing accurate, up-to-date information to health care providers, immunization registries (confidential, computerized information systems that contain information about immunizations and children) can make it easier to carry out the demonstrably effective immunization strategies. All states are currently in the process of establishing population-based immunization registries containing information on all children within their borders.

In the United States, infants and children may receive immunizations from private providers (typically in conjunction with other well-child services) or from public sector sites such as local health departments (in which case immunizations might be the only services provided) or community health centers. Traditionally, vaccines provided in the public sector have been free, whereas private providers have charged for the vaccines. Consequently, lower-income families typically went to public sector facilities to receive vaccine, even though they might have been using a private physician for other care. Until the middle of the 1990s, it was estimated that approximately one-half of all U.S. children received immunizations from private providers and one-half from the public sector. Enactment of the Vaccines For Children (VFC) program in 1994 made free vaccine available to private providers for use in uninsured or under-insured children and led to a major shift in immunization provision. In 1998, approximately 70 percent of all childhood vaccines were administered in the private sector and 30 percent in the public sector, meaning that more children were receiving immunizations in their "medical home" than had been the case previously.

IMMUNIZATIONS WORLDWIDE

Since 1979, the World Health Organization (WHO) has coordinated an Expanded Program on Immunization (EPI), which seeks to bring vaccines against six diseases—diphtheria, measles, pertussis (whooping cough), poliomyelitis, tetanus, and tuberculosis—to all children in the world. An abbreviated immunization schedule has been developed that calls for a dose of BCG (Bacille Calmette-Guerin) at birth; three doses of DTP (combined diphtheria and tetanus toxoids and pertussis vaccine) and OPV (oral polio vaccine) given at six, ten, and fourteen weeks of age; and a single dose of measles vaccine at nine months of age. BCG protects infants against severe forms of tuberculosis (such as tuberculous meningitis) but does not alter the overall transmission of tuberculosis.

The EPI succeeded in reaching immunization coverage levels of approximately 80 percent in the world's children by 1990 (the year of the Children's Summit), but levels have been relatively stagnant since that time, even decreasing in some areas. Coverage varied markedly among (and within) countries. Some of the reasons for the lack of further progress include: the overall economic situation in many countries, the fragile nature of the countries' health services, lack of political support, and problems in management of immunization programs. In 1991 a recommendation was made to administer hepatitis B vaccine to all children (three doses: at birth, six weeks, and fourteen weeks; or along with the DTP vaccine) but this has not been widely implemented in most developing countries. Introduction of other (newer) vaccines such as Hib is problematic. These vaccines are considerably more expensive than traditional vaccines, there are few manufacturers (sometimes only one, as a result of innovation and patent protection), and purchase of vaccines may require hard currency, which may be difficult for some developing countries to obtain. The development of the Global Alliance for Vaccines and Immunization and the Global Children's Vaccine Fund in early 2000 give hope that mechanisms may be developed to facilitate introduction of important new vaccines in developing countries.

ERADICATION OF VACCINE— PREVENTABLE DISEASES

Global eradication of smallpox in the late 1970s is probably the greatest single achievement in health to date. Although both William Jenner and Thomas Jefferson predicted eventual eradication at the end of the eighteenth century, it took nearly two hundred years to accomplish. The intensive global effort for eradication began in 1967 with the result that the last naturally occurring case of smallpox occurred in 1977. The World Health Assembly certified eradication in 1980. The initial strategy to achieve eradication was mass vaccination of the population, but over time this was refined to a strategy of search and containment— search for cases of smallpox and containment of transmission through vaccinating all persons who might have been exposed in a geographic area.

An effort is currently underway to eradicate polio from the world by the end of 2000. The strategy for eradication involves attaining high levels of coverage with routine vaccination with OPV, special immunization campaigns, and vigorous surveillance to detect and investigate possible cases of polio. The special immunization campaigns typically occur as National Immunization Days, semiannual events in which all children in the country less than five years old are given OPV on a single day, regardless of their previous vaccination status. Significant progress is being made: no locally arising cases of polio have occurred in the Americas since 1991, none in the Western Pacific Region of the World Health Organization (including China) since 1997, and none in the European Region since 1998. At the beginning of 2000, the major problems remaining were in South Asia and sub-Saharan Africa. Whether the target will be met on schedule is not clear. It is clear that eradication is technically feasible—the uncertainties relate to political will and financial support.

Other diseases that are potential candidates for eradication through appropriate use of vaccines include measles, mumps, and rubella. Measles is the most serious of these, still accounting for nearly 900,000 deaths a year (half of them in sub-Saharan Africa), and there is substantial support for consideration for elimination or eradication. The public health impact of rubella and mumps is not as widely recognized and there is not the same degree of enthusiasm for their eradication, although it is estimated that more than 100,000 cases of congenital rubella syndrome occur each year around the world. Although all three conditions could be attacked simultaneously by using MMR (combined measles-mumps-rubella) vaccine, the additional vaccine costs would be substantial.

FUTURE VACCINES

Recent advances in biotechnology and understanding of the immune process make it likely that the pace of vaccine development and introduction will accelerate. Although this will mean that there is greater opportunity for prevention of disease and death, it will have additional consequences, such as increasing complexity of the immunization schedule and the need for additional injections. Development of combination vaccines can help alleviate this problem but, since there is at least a theoretical issue of incompatibility and interference between different vaccines, each combination must be tested thoroughly before it can be approved. Additionally, the prospective availability of combined vaccines from different manufacturers with slightly different components may add further complexity to the schedule and to decision making about what a given individual needs.

The biotechnology revolution has made it possible to explore novel approaches to immunization, such as incorporating into other microorganisms the antigens that elicit protective antibodies (another way of making combination vaccines) or even incorporating antigens into foodstuffs such as potatoes or bananas. Additionally, the prospect of administering vaccines by aerosol or using transdermal patches is being investigated, as is the possibility of using purified DNA from the causative organism as the means to induce immunity. Because of the potential for transmission of infectious diseases (e.g., hepatitis B, HIV/AIDS) through reuse of needles or inadvertent needle-sticks, disposal of needles has become a significant problem and has led to the development of "auto-destruct" syringes and needles that cannot be used more than once. Most designs to date do not prevent inadvertent needlesticks, however. Consequently, needleless approaches to administration are being pursued, including pressure injection of liquid or powder vaccine, aerosol/inhalation, and use of transdermal absorption.

CONCLUSION

Immunizations have been among the most successful public health interventions to date. Through appropriate use of vaccines, smallpox has been eradicated from the earth, poliomyelitis is on the verge of eradication, and there have been dramatic reductions in morbidity and mortality due to with many other diseases. Recent scientific advances give promise that even more diseases can be brought under effective control. A remaining challenge is to ensure that all people of the world benefit from immunizations.

Bibliography

Centers for Disease Control and Prevention (1999). "Achievements in Public Health, 1900–2000: Impact of Vaccines Universally Recommended for Children; United States, 1900–1998." Morbidity and Mortality Weekly Report 48(12):243–248.

—— "Recommendations of the Advisory Committee on Immunization Practices." Available online at http://www.cdc.gov/nip/publications/ACIP-list.htm.

Offit, P. A., and Bell, L. M. (1998). What Every Parent Should Know About Vaccines. New York: MacMillan.

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immunization

immunization Immunization is the process of conferring increased resistance (or decreased susceptibility) to infection. The term ‘vaccination’ is also used to describe this kind of protective measure, although, strictly speaking, this term refers only to the protection conferred against smallpox by material taken from a cow infected with vaccinia virus (which causes cowpox). Inoculation also is used synonymously for immunization, but less commonly nowadays.

The history of immunization goes back to early attempts to prevent smallpox by the Chinese; much later, in the eighteenth century, came the classical experiments of Edward Jenner in Gloucestershire, who induced protection in a child by the inoculation of material from a cow infected with cowpox.

Achievements in the history of immunization are summarized in Table 1. Although the early work to control infection was made before microbiological methods were firmly established, rapid progress was made, based on sound scientific principles, once modern bacteriology, and later virology, came on to the scene. For example, the isolation of poliovirus allowed for the development by Jonas Salk, and later by Albert Sabin in the 1950s, of highly effective poliovaccines, which led to a dramatic diminution in poliomyelitis. Before then, there were alarming outbreaks of this paralytic disease: over 8000 cases occurred in the UK in 1950. By the late 1980s, poliovirus capable of producing paralysis was still circulating widely in all continents of the world except Australia. But by 1998 the Americas were polio-free and elsewhere there is substantial progress being made towards the goal of worldwide eradication of this much dreaded disease.

Similarly, with measles the isolation of measles virus in 1954 made it possible to culture a strain which is now the basis of the measles vaccine in use today. Prior to the use of the vaccine, in the UK as many as 800 000 cases were notified annually, but its introduction has resulted in a dramatic decline.

Introduction into Britain from Turkey by Lady Wortley Montagu of inoculation of material

from smallpox patients into healthy persons (variolation)

1796

First vaccination against smallpox performed by Jenner

1880

Pasteur developed fowl cholera vaccine

1881

Pasteur, Roux, and Chamberland introduced anthrax vaccine

1885

Pasteur developed rabies vaccine

1895

Yersin produced plague vaccine

1898

Almroth Wright developed typhoid vaccine

1921

Calmette and Guérin introduced BCG vaccine

1923

Ramon developed diphtheria toxoid

1927

Ramon and Zoeller developed tetanus toxoid

1940

National immunization campaign launched in Britain by Ministry of Health; did not become

widespread until 1942

1954

Salk (killed) polio vaccine introduced

1957

Sabin (live) polio vaccine introduced

1960

Measles vaccine developed by Enders

1962

Rubella vaccine developed by Weller

1967

Jeryl Lynn strain of live attenuated mumps vaccine licensed in the US

1968

Meningococcal (type C) vaccine developed

1968

Measles vaccine introduced on a national scale in Britain

1970

Rubella vaccine became available in Britain

1981

Hepatitis B vaccine licensed in US

1988

Measles, Mumps, Rubella (MMR) vaccine introduced into Britain

1992

Haemophilus influenzae b (HiB) vaccine introduced into Britain

Immunization is one of the most cost-effective public health measures available. But although it is possible to manufacture vaccines against a wide variety of viruses and bacteria, it is, of course, important to ensure that the introduction of a particular vaccine will always confer a major benefit to the population receiving it. Therefore certain broad principles are followed before a vaccine is recognized as being suitable for general use: (i) there should be a major risk of contracting the infection against which the vaccine is intended to protect; (ii) the vaccine should prevent an illness which (including complications and sequelae) is regarded as serious and especially if it can be fatal; (iii) the efficacy of the vaccine should be sufficiently high; (iv) any risk associated with the vaccine should be sufficiently low; (v) the procedures and the number of doses required for successful immunization should be acceptable to the public.

An ideal vaccine should confer long-lasting, preferably lifelong, protection against the disease; it should be inexpensive enough for large scale use, stable enough to remain potent during transportation and storage, and have no adverse effect on the recipient. If the introduction of a vaccine is agreed upon at national level then a further decision has to be made as to whether it should be for general use (e.g. polio vaccine) or for specific use when exposure is possible (e.g. typhoid vaccine, given when travelling to regions where typhoid is endemic).

Vaccines may induce immunity against infection either actively or passively.

Active immunization

Active immunization is brought about by stimulating the individual's own immunity by introducing either inactivated (killed) or attenuated (live, but enfeebled) agents (Table 2). The protective response by the body is mainly expressed through: (i) specific antibodies, measurable by serological tests, which confer protection against many agents, particularly viruses and toxins. (ii) the cellular immune response, which involves both phagocytes and ‘memory cells’.

Inactivated vaccines

are prepared in three ways (examples in Table 2): (i) from killed whole organisms; (ii) from sub-units of the killed organisms; (iii) from the toxins which the organisms release, inactivated by formaldehyde (toxoids).

When the organisms have been killed there can be no multiplication within the body, and thus these vaccines cannot produce infection similar to the natural disease. On the other hand, local and whole body reactions may result from response to the organism or to foreign protein used in the vaccine. If the person has not previously been immunized, more than one dose is usually required, although some response can be produced by even a single dose. Protection often lasts for many years, although periodic ‘boosts’ by subsequent injections may be required to maintain immunity.

Attenuated vaccines

are prepared from modified strains of the causal organisms or from related organisms. Because of this, some live vaccines may sometimes cause illness resembling the natural disease, but the symptoms are usually milder. In general, however, these vaccines have fewer side-effects than inactivated ones and the immunity usually lasts for many years.

Passive immunization

Passive immunization is obtained by giving pre-formed, antibodies. These are usually injected in the form of human immunoglobulin or, rarely, antisera prepared in animals. Protection is usually rapid, but the immunity derived is often short-lived, being limited to the time taken for the antibodies to be broken down in the body — from a week or so, with animal antisera, to about six months for protection against hepatitis A by human normal immunoglobulin.

Special risk groups

include those persons particularly liable to suffer from complications of infection, for whom protection by appropriate immunization is therefore of particular importance: for example, those with chronic lung disease, asthma, congenital heart disease, Down's syndrome, or Human Immunodeficiency Virus (HIV) infection, and babies who are born prematurely or are ‘small-for-dates’. Immunization of travellers to some countries overseas is often a particular problem, as the risk of certain infections may be especially high and it often has to be given when time is short.

Surveillance of immunization

procedures is necessary. Immunization it is not without its occasional hazard and it is important that those involved should balance the risk of the disease against the possible risk of the vaccine. Surveillance measures should be aimed at assessing not only the application, utilization, and effectiveness of vaccines in the control of infection, but also any side effects, so that rational decisions about whether to vaccinate can be made.

In conclusion, the achievements of successful immunization policies have been spectacular when the ravages caused by vaccine-preventable infections in former years are compared with those of today. Smallpox has now been eradicated, and other greatly feared infections (such as poliomyelitis) are well under control. Because immunization can often be given quite cheaply and quickly to large numbers of people, it is a remarkably cost-effective measure, which has undoubtedly made a major (if not the major) contribution to the overall protection of the world's population against infection.

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Immunization and Children's Physical Health

Encyclopedia of Education
COPYRIGHT 2002 The Gale Group Inc.

IMMUNIZATION AND CHILDREN'S PHYSICAL HEALTH

With the exception of safe water, no other public health intervention has had a greater impact in reducing deaths related to infectious disease than vaccinations. Smallpox was eradicated in 1977; wild-type poliomyelitis was eliminated from the Western hemisphere in 1991. Among children under five, measles and invasive Haemophilus influenzae type b (Hib) have both been reduced to record low numbers. Deaths associated with smallpox, diphtheria, pertussis, tetanus, paralytic poliomyelitis, measles, mumps, rubella, congenital rubella syndrome, and Hib decreased an average of nearly 100 percent during the twentieth century.

Though the United States is reaching record low levels of vaccine-preventable disease, continued immunization is important. Pathogenic viruses and bacteria still circulate in the United States, and with continuing globalization, the threat of disease spread increases. Immunization protects the general population from disease; individuals who are immunized have inherent protection from disease while those who are not immunized are protected by the limited likelihood that they will be exposed to another unimmunized and infected individual. Furthermore, immunization protects a population from disease; vaccinated individuals acquire immunologic protection from pathogens for themselves. As a consequence, immunized individuals are protected secondary to a decreased incidence of disease. This phenomenon of herd immunity, however, will not be achieved unless 80 percent to 95 percent of the population is immunized.

Childhood Immunizations

A detailed childhood immunization schedule is released yearly by the National Immunization Program (NIP), the American Academy of Pediatrics (AAP), and the American Academy of Family Physicians (AAFP). This schedule informs parents and health care providers which vaccines children need to receive and when they should receive them. An updated schedule can be found at the website of the Centers for Disease Control and Prevention.

State governments are responsible for passing and enforcing school immunization laws. Currently all fifty states have immunization requirements for children entering school. The vaccinations required may be different for different states. Additionally, states may also differ in the number and types of philosophical or religious exemptions they allow for children entering school. Vaccination decreases the risk of infection and outbreaks in schools by reducing the number of unprotected people who may be infected that are capable of transmitting the disease within their schools and communities.

Immunizations are available either free of charge or for a reduced cost at local health departments for children whose parents cannot afford to take their children to private physicians for immunization. Furthermore, on August 10, 1993, the Omnibus Budget Reconciliation Act (OBRA) created the Vaccines for Children Program (VFC) program as Section 1928 of the Social Security Act in order to increase access to immunizations. The program began on October 1, 1994; it provides vaccinations at no cost to VFC-eligible children seeing either public or private providers.

Vaccine-Preventable Diseases

The following eleven vaccine-preventable diseases are addressed through standard vaccination programs. A number of these diseases are covered by mandatory school immunization laws.

Diphtheria is a respiratory disease caused by a virus that is spread by coughing and sneezing. Symptoms include sore throat and low-grade fever. Left untreated, diptheria can lead to airway obstruction, coma, and death. Vaccines that contain the diptheria toxoid include the DTP, DtaP, DT, and Td vaccines.

Tetanus (lockjaw) is a disease of the nervous system that is caused by bacteria that enters the body through a break in the skin. Early symptoms include lockjaw, stiffness in the neck and abdomen, and difficulty in swallowing. Later onset symptoms include fever, elevated blood pressure, and severe muscle spasms. Death occurs in one third of all cases, especially among the elderly. Tetanus toxoid is also contained in the DTP, DT, DtaP, and Td vaccines.

Pertussis (whooping cough) is a highly contagious bacterial respiratory disease spread through coughing and sneezing. Symptoms include severe fits of coughing that may interfere with eating, drinking, and breathing. Pertussis may cause pneumonia, encephalitis, and infant death. The pertussis vaccine is contained within the DTP and DtaP vaccines.

Haemophilus influenzae type b is a bacterial infection that predominantly affects infants. It is spread by coughing and sneezing. Symptoms include skin and throat infections, meningitis, pneumonia, sepsis, and arthritis and may be severe for children under the age of one. Risk of disease is reduced after the age of five. A Hib vaccine can prevent the disease.

Hepatitis A is caused by the Hepatitis A virus and is spread most commonly through the fecaloral route when the stool of an infected person is put into another person's mouth. It can also be transmitted by ingesting food or water that contain the virus. The disease affects the liver. Symptoms are unlikely, but if present they may include yellow skin or eyes, fatigue, stomach ache, loss of appetite, and nausea. This disease is prevented using the Hepatitis A vaccine.

Hepatitis B is caused by the Hepatitis B virus and is spread through sexual contact or through contact with the blood of an infected person. As with the initial clinical presentation of Hepatitis A, infection with Hepatitis B may follow an indolent course and manifest no symptoms. The likelihood of developing Hepatitis B increases with age. If present, symptoms are similar to Hepatitis A. The Hepatitis B vaccine prevents the disease.

Mumps is caused by a virus and is spread through coughing and sneezing. It is a disease that affects the lymph nodes. Symptoms include fever, headache, muscle ache and swelling of the lymph nodes close to the jaw. Infection with the mumps virus may also lead to meningitis, inflammation of the testicles or ovaries, inflammation of the pancreas and permanent deafness.

Measles is a highly contagious respiratory disease caused by a virus that is transmitted through coughing or sneezing. Symptoms include rash; high fever; cough; runny nose; and red, watery eyes lasting about a week. The measles may cause diarrhea, ear infections, pneumonia, encephalitis, seizures, and death. The measles vaccine is contained within the MMR, MR and measles vaccines.

Rubella (German measles) is a viral respiratory disease spread through coughing and sneezing. Symptoms may include a mild rash and fever for two to three days in children and young adults. Complications are severe for pregnant women, whose children frequently have congenital birth defects. The rubella vaccine is contained within the MMR, MR, and rubella vaccines.

Polio is a viral disease of the lymphatic and nervous systems. Transmission occurs through contact with an infected person. Symptoms include fever, sore throat, nausea, headaches, stomach aches, and stiffness in the neck, back and legs. OPV and IPV are the vaccines in current use.

Varicella (chickenpox) is a highly contagious disease caused by bacteria and is spread by coughing or sneezing. Symptoms are a skin rash of blister-like lesions on the face, scalp, or trunk. The varicella vaccine prevents this disease.

Immunization Safety

Before vaccines are approved by the Food and Drug Administration, they undergo rigorous scientific testing to ensure that they are safe and effective. However, differences in each individual's response to an antigenic vaccine challenge account for the rare occurrences ranging from vaccine failure to anaphylaxis. The National Childhood Vaccine Injury Act (NCVIA) of 1986 created the National Vaccine Program Office within the Department of Health and Human Services. It requires that all providers who administer vaccines provide a Vaccine Information Statement that explains the disease and the risks and benefits of vaccination to the vaccine recipient, parent, or legal guardian. The NCVIA also created the Vaccine Adverse Event Reporting System, a mandatory reporting system for health care providers. The National Vaccine Injury Compensation Program was also created under the NCVIA as a no-fault system for compensating people injured by vaccination.

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Immunization

World of Microbiology and Immunology
COPYRIGHT 2003 The Gale Group Inc.

Immunization

When a foreign disease-causing agent (pathogen) enters the body, a protective system known as the immune system comes into play. This system consists of a complex network of organs and cells that can recognize the pathogen and mount an immune response against it.

Any substance capable of generating an immune response is called an antigen or an immunogen. Antigens are not the foreign bacteria or viruses themselves; they are substances such as toxins or enzymes that are produced by the microorganism. In a typical immune response, certain cells known as the antigen-presenting cells trap the antigen and present it to the immune cells (lymphocytes). The lymphocytes that have receptors specific for that antigen binds to it. The process of binding to the antigen activates the lymphocytes and they secrete a variety of cytokines that promotes the growth and maturation of other immune cells such as cytotoxic T lymphocytes. The cytokines also act on B cells stimulating them to divide and transform into antibody secreting cells. The foreign agent is then either killed by the cytotoxic T cells or neutralized by the antibodies.

The process of inducing an immune response is called immunization. It may be either natural, i.e., acquired after infection by a pathogen, or, the immunity may be artificially acquired with serum or vaccines.

In order to make vaccines for immunization, the organism, or the poisonous toxins of the microorganism that can cause diseases, are weakened or killed. These vaccines are injected into the body or are taken orally. The body reacts to the presence of the vaccine (foreign agent) by making antibodies. This is known as active immunity. The antibodies accumulate and stay in the system for a very long time, sometimes for a lifetime. When antibodies from an actively immunized individual are transferred to a second non-immune subject, it is referred to as passive immunity. Active immunity is longer lasting than passive immunity because the memory cells remain in the body for an extended time period.

Immunizations are the most powerful and cost-effective way to prevent infectious disease in children. Because they have received antibodies from their mother's blood, babies are immune to many diseases when they are born. However, this immunity wanes during the first year of life. Immunization programs, therefore, are begun during the first year of life.

Each year in the United States, thousands of adults die needlessly from vaccine-preventable diseases or their complications. Eight childhood diseases (measles , mumps , rubella, diphtheria , tetanus , pertussis , Hemophilus influenzae type b, and polio) are preventable by immunization. With the exception of tetanus, all the other diseases are contagious and could spread rapidly, resulting in epidemics in an unvaccinated population. Hence, vaccinations are among the safest and most cost-efficient public health measures. Vaccinations against flu (influenza ), hepatitis A, and pneumococcal disease are also recommended for some adolescents and adults. The vaccines indicated for adults will vary depending on lifestyle factors, occupation, chronic medical conditions and travel plans.

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immunization

immunization The production of immunity in an individual by artificial means. Active immunization (vaccination) involves the introduction, either orally or by injection (inoculation), of specially treated bacteria, viruses, or their toxins to stimulate the production of antibodies (see vaccine). Passive immunization is induced by the injection of preformed antibodies.

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immunization

immunization Practice of conferring immunity against disease by artificial means. Passive immunity may be conferred by the injection of an antiserum containing antibodies. Active immunity involves vaccination with dead or attenuated (weakened) organisms to stimulate production of specific antibodies and so provide lasting immunity.

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immunization

immunization (im-yoo-ny-zay-shŏn) n. the production of immunity by artificial means. Passive immunity may be conferred by the injection of an antiserum, but the production of active immunity calls for the use of a vaccine (see vaccination). see Immunization schedules www.immunisation.nhs.uk The NHS immunization website

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